CA-133656: iops is always (nearly) zero

Problem: The iops values are always or at least nearly zero

Root Cause: rrdd_plugin_iostat query those values from /sys/block/tdX/stat. It treated the 1st (zero-based) and 5th columns as the the io-read-requests and io-write-requests values. However, according to https://www.kernel.org/doc/Documentation/block/stat.txt, 1st/5th are io-read-merged and io-write-merged and 0th/4th are io-read-requests and io-write-requests.

Solution: Revise the indices.